We continue to study placental proteins as cancer markers. Ectopic production of alpha subunit in vitro by Chang liver cells is differentially regulated by butyrate and by dibutyryl cyclic AMP. Alpha production in vitro by HeLa-fibroblast human cell hubrids is specifically correlated with malignancy but production of placental alkaline phosphatase showed no such correlation. Partially deglycosylated SP1 (79% carbohydrate removed) has an apparent molecular weight of 30,500. Placental protein 5 production by fibroblasts is regulated differently from SP1 production by these cells. Placental proteins are not useful in distinguishing malignant from benign plasma cell dyscrasias or malignant from benign breast lesions.